The present invention relates generally to an elegantly simple, low-cost electronic control to improve the efficiency of air conditioners or heat pumps with fixed speed evaporator blower, when the system is operating in cooling mode. It will also provide a benefit for a heat pump operating in heating mode where the vapor compression system is providing the heating function and the indoor coil is now the condenser rather than the evaporator as it was in cooling mod. A fixed speed indoor air handler blower motor of the air conditioner or heat pump is typically manufactured with multiple speed taps that allow the installer to selected one of several possible speeds for the blower to operate when the unit is installed. Typically, these motors have three speeds, but some motors have five or more possible speeds, and of course, a motor blower could be manufactured with only two speeds. Rather than operate this indoor air handler blower at a single speed, the present invention varies the speed of the motor, thereby varying the airflow with a low cost motor instead of using a far more expensive variable airflow technologies such as an electrically commutated motor, variable speed drive, or complex control and sensor logic.
It is well known in the art that increasing the rotational speed of an evaporator blower of a vapor-compression cooling system (thereby increasing the air flow) reduces the air temperature change for a constant cooling capacity and thereby can increase the operating temperature of the evaporator. An increase in evaporator temperature results in a lower temperature lift, and therefore an increased cooling capacity and/or lower compressor power draw. However, the power consumption of the fan increases as well. Therefore, if the increase in temperature and resulting reduction in compressor power draw does not offset the increase in power draw and heat dissipation by the blower used to increase rotational speed of the blower, the net effect is an increase in power consumption and reduction in the Coefficient of Performance (commonly referred to as COPc).
The COPc is the cooling capacity at a particular operating condition (indoor and outdoor wet and dry bulb temperatures) divided by the power consumption at those conditions. The power consumption is mainly comprised of the blower motor power draw, condenser fan power draw, and compressor power draw. Another common term used is Energy Efficiency Rating (or EER). Like COPc, the EER is a ratio of cooling capacity divided by power. However, for the EER calculation, the cooling capacity is measured in units of BTU/hr, and the power input is measured in watts. Although it is not a common engineering practice to have dissimilar units, this EER metric has found wide acceptance and there is a simple conversion which can be calculated by adjusting for the units mismatch, namely EER=3.4 times the COPc. Finally, there is a seasonally adjusted EER which represents a seasonal average of the EER and is referred to as SEER.
The basic concept that higher evaporator temperature improves cycle efficiency comes from the fundamental Carnot cycle and basic principles of thermodynamics. Higher blower speeds means more air flow across the evaporator coil, improved heat transfer and a higher coil temperature and therefore reduced thermal lift. Reduced thermal lift means lower compressor work and increased capacity. However, we have realized that from a systems perspective, increasing blower speed and airflow (and thus, increasing evaporator saturation temperature) will not always increase system efficiency (COPc) because increased blower speed increases blower power draw. The power input to the blower has a heating effect on supply air, meaning increased blower power draw will have an additional heating effect on the conditioned air (decreasing the numerator in the COPc calculation) and increases the system power draw (increasing the denominator in the COPc calculation). For COPc to increase due to increased blower airflow, the additional cooling capacity from the reduced temperature lift must be more significant than the increased heating effect and power draw realized from increasing blower speed. Whether the COPc is increased or decreased due to increased blower speed is a function of operating point (ambient and return air conditions), system design, blower type, ducting, and air filter selection and condition. We have found that by merely using the evaporator saturation temperature, or another variable which parallels that temperature, such as evaporator pressure, evaporator surface temperature, or evaporator air discharge temperature, the evaporator blower motor speed can be altered to reduce overall power consumption and thereby improve COPc, EER, and SEER in a totally unanticipated and surprising manner.
As stated above, the basic concept that higher evaporator temperature improves cycle efficiency comes from the fundamental Carnot cycle and basic principles of thermodynamics. There are numerous references to this well-known fact, such as U.S. Pat. No. 5,303,561 col. 2, lines 13-1, which states that “this is due to the fact that a highly efficient air-conditioning system nominally operates at higher evaporator coil temperature . . . . ” That patent proposes the use of a continually variable fan to modulate the fan speed using a “integrated drive and variable speed motor” (col 6, lines 21-22) and this approach in one form or another is used along with complex control logic to establish the optimal fans speed for the desired temperature and humidity in the building using a combination of indoor air temperature, indoor air humidity and outdoor air temperature for selecting the indoor evaporator blower speed.
Likewise, U.S. Pat. No. 6,282,910 discusses using an AC induction blower motor along with a variable speed drive, where alternating current power is directly coupled to the motor at nominal line frequency for full speed operation, or an inverter output is used to alter blower speed when reduced blower speed is desired. Although the existing alternating current (AC) induction motor is used, a continuously variable speed drive inverter is required to vary the motor speed.
Others have proposed modulating the compressor speed (see, e.g., U.S. Pat. No. 7,946,123), but these known approaches also required variable speed drive inverters or different compressors to implement in a retrofit configuration that is both costly and impractical. Likewise, U.S. Pat. No. 7,739,882 discloses a variable speed control system for use with a variable speed compressor.
Significantly, the present invention uses the existing multi-speed selection capability of fixed speed air handler blowers used by most manufacturers (that are not employing the more expensive continuously variable speed blowers). These fixed-speed blowers have several different winding combinations that provide to the HVAC installer the ability to select from an assortment of fixed speeds from the same blower motor (depending on the wiretap that is activated), to best balance the air flow for a particular installation. In a normal application, once the speed is selected for a particular operating mode (cooling, or heating), the air handler blower operates at this speed setting whenever the motor is activated. Typical air handler blower motors have three to five speeds as above noted. Blower speed is typically selected by placing the power-leads on the quick-disconnect post that correlates to the desired blower speed or connecting the power-leads to specific electrical wires originating from the motor and are differentiated by color. Once a speed is selected, the air handler blower will operate at the selected speed whenever it is powered in that mode. While some systems allow for a single fixed speed in cooling mode and a potentially different speed in heating mode, once heating or cooling operation is selected, the indoor coil's blower motor operates at a fixed speed determined by the technician who installed or maintains the system. Instead, the present invention uses at least two of the existing multi-speed blower motor taps to provide dynamic variable speed adjustments during air conditioning operation (cooling mode) based on a single input such as evaporator saturation temperature, evaporator saturation pressure, evaporator outlet air temperature or evaporator surface temperature. One skilled in the art would also understand from this disclosure that for heat pump applications, where the same indoor blower motor is now blowing air across the indoor coil which is functioning as a condenser in the heating mode, the same temperature sensor, or the like, is measuring condenser saturation temperature and can provide dynamic speed variation, this time for the condenser cooling, to improve overall performance in heating mode using the same device, with the only modification being to use the absolute temperature difference of the temperature, as discussed later in this disclosure.
U.S. Pat. No. 7,191,607 discloses a speed control that selectively operates the fixed speed blower motor to slow the speed of the blower for dehumidification but only in the initial stages of the cooling mode, typically the first 5 to 7 minutes. This approach did not, however, recognize that blower speed should be modified to improve system efficiency, and that such blower speed modulation could be easily achieved by actuating different motor windings to produce higher performance with substantially less complexity.
The present invention uses the exact opposite control logic on start-up when compared to the approach in the above-described U.S. Pat. No. 7,191,607. In the present invention, when the air conditioner is started and the evaporator blower is activated, the blower speed is set to maximum speed for the initial startup rather than a slow speed in order to determine the highest possible evaporator operating temperature as the initial baseline, from which the effect of slower evaporator blower (fan) speeds on evaporator temperature can be determined.
Our discovery lowers overall, energy consumption by lowering the air flow (blower fan speed), for those situations when no significant increase in compressor power is observed (as determined by an decrease in evaporator temperature or increase in condenser temperature) due to the lower air flow across the specific heat exchanger coil. A lower blower speed without an increase in compressor power results in a reduction in overall power consumption, and therefore a boost in performance (COP, EER, or SEER). That is, a lower evaporator blower speed does not result in significant decrease in evaporator saturation temperature and/or a lower condenser fan speed (heat pump in heating mode, the condenser fan speed is the indoor blower motor speed) does not result in an increase in condenser saturation temperature. The further benefit of the present invention is simple installation, and the ability to use the existing blower motor, thermostat and overall control system that activates the system. As stated, this novel approach can, of course, also be extended to condenser fan speed, where the condenser fan speed is lowered when no appreciable increase is compressor power occurs (no appreciable increase in condenser temperature) as a result of the lower condenser fan speed, notwithstanding the fact that condenser fans with multiple speed taps are not common in existing air conditioning systems. However, as one well versed in the art would understand, for heat pump systems operating in heating mode, the condenser is the indoor coil (and the evaporator is the outdoor coil), so for a heat pump operating in heating mode, the indoor blower motor is the condenser fan and therefore multiple speed fan motors are possible.
The present invention is elegant in its simplicity and ability to provide significant improvements in performance (COP, EER and SEER) without replacing the existing thermostat based control system, the existing evaporator blower motor, or adding a complex, costly and large inverter or alternative speed controller system. Due to its simplicity, the invention can be easily and quickly retrofitted into exiting air conditioning and heat pump cooling systems in the form of a control board.
More specifically, one embodiment of the invention uses a single electronic control board to boost the system performance. This board is located either inside or outside the air handler, within practical reach to the blower motor (indoor air handler motor). The control board can be powered by 24 VAC from a transformer inside the air handler or the AC voltage (115-240 VAC for example) that powers the blower motor. The two power leads that were originally connected to the blower motor (when in cooling mode) are connected to the control board to indicate when the blower motor should be operating in cooling. Typically, for the cooling mode variable speed control as many as six power leads (corresponding to five speeds and a common) or as few as three power leads (corresponding to a maximum and minimum blower speed and the common), are connected from the control board of the present invention to the different speed setting posts on the air handler blower motor.
In a currently preferred embodiment of the invention, the control board has four connections for two inputs. One input, uses two connections (2 wires) to a device to measure temperature such as a Thermistor, thermocouple, RTD or the like to measure evaporator surface temperature (sensor input). Evaporator saturation pressure, saturation temperature or outlet air could be measured instead of the evaporator surface temperature. The remaining input consists of the two blower power leads that would normally be connected directly to the blower motor (if this invention was not being used) to power the blower motor, when the vapor compression system is operating in cooling mode. The two power leads are a common wire and the switched hot lead for powering the blower when the vapor compression system is operating. These two input power wires are removed from the blower motor and attached to the control board of this invention, and the temperature sensor is attached to the surface of the indoor coil (evaporator in cooling, condenser for heat pump in heating mode). The sensor is preferably located in a region of the evaporator that should contain saturated refrigerant such as in the region of the evaporator directly downstream of the expansion device.
The control board of the present invention has outputs which are connected to three speed taps of the blower motor and the common power connection. The High Speed power lead is connected to the high-speed post of the multi-tap motor, the Medium-Speed power lead is connected to the medium-speed post of the multi-tap motor, and the Low-Speed power lead is connected to the low speed post of the multi-tap blower motor. The common power lead is connected to the common tap of the blower motor. The control board of our preferred embodiment of the invention is powered by scavenging power from the input power leads when they are active, that is when they are sending power to the blower motor. If the blower motor has more than three speeds, then the High-Speed tap is connected to the highest speed tap, the Low-Speed tap is connected to the lowest speed tap and the Medium-Speed tap is connected to one of the speeds nearest the middle speed of that motor. If the motor only has two speeds, then both the Low- and Medium-speed taps are connected to the slower of the two speeds.
Of course, it is well known in the art that the control board can also be powered by the input line power or the 24 VAC available from the transformer inside the air handler. The remainder of the air conditioning or heat pump control system is unchanged.
In our currently preferred embodiment, however, when the thermostat on the air conditioning system calls for cooling, power is supplied to the compressor(s), the condenser fan(s) and the evaporator blower(s). The power is sent to the evaporator blower motor or motors via the aforementioned two power leads which in our invention are now connected to the control board and provide power to the latter. Likewise, in our currently preferred embodiment, however, when the thermostat on a heat pump system is calling for heating or cooling and power is supplied to the compressor(s), the outdoor heat exchanger fan and the indoor coil's blower motor, the power is sent to the indoor blower motor or motors via the aforementioned two power leads which in our invention are now connected to the control board and provide power to the latter.